1. Field of the Invention
The present invention relates to a color cathode ray tube, and more particularly, to a shadow mask for a color cathode ray tube using Aluminum Killed (AK) which has improved brightness and bright uniformity at corners, reduced doming effect, enhanced curved surface maintenance strength and lowered production cost.
2. Background of the Prior Art
In general, the cathode ray tube is the display apparatus that converts electrical signals into electron beams and scans the fluorescent screen with the electron beams to generate visible light and thus display images. Such cathode ray tubes are the most popular display apparatus since they are very excellent in the ratio of display quality to cost.
FIG. 1 shows schematically a structure of a conventional color cathode ray tube. Referring to FIG. 1, the conventional cathode ray tube includes a panel 1 having its outer surface that is flat or has some curvature, and a funnel 2 coupled sealingly with the panel 1 to form a tube. The panel 1 and the funnel 2 are coupled sealingly with a frit glass. Since the tube including the panel 1 and the funnel 2 keeps its inside in a vacuum, it may explode due to an external impact, which is very dangerous. In order to prevent this, a strengthening band 12 is added to a contact portion of the panel 1 and the funnel 2.
The funnel 2 consists of a neck 10 whose shape is narrow tube and a cone portion that expands at the end of the neck 10. The neck 10 is provided with electron guns to project electron beams 11. The cone portion is provided with a deflection yoke 9 to deflect the electron beams 11. On the other hand, the colors of the electron beams deflected by the deflection yoke 9 are selected by a shadow mask 5 that has fine holes. The electron beams are shot across a fluorescent material coated on the inside of the panel 1, so that the fluorescent material emits lights of each color, red, green and blue. To achieve this, the shadow mask 5 has a lot of holes and each of the fluorescent materials for red, green and blue is coated on the fluorescent surface corresponding to the holes. The electron beams are shot across the fluorescent material to emit light and images are displayed on the fluorescent surface. The shadow mask is supported by a frame 4 to be spaced with the panel 1. The support spring installed by the frame 4 is coupled with a stud pin 6 mounted on the panel 1 to be supported firmly in the tube. To prevent the electron beams 11 from shifting due to an external magnetic field, an inner shield 7 made of magnetic material is coupled with the frame 4 to be supported.
The operation of the general color cathode ray tube will be described. The electron guns 8 generate electron beams 11 and the electron beans 11 are shot across the fluorescent surface inside the panel 1 by a voltage applied to the cathode ray tube. In this time, the electron beams 11 are deflected by the deflection yoke 9. Each color of the beams is selected by the shadow mask 5. The electron beams 11 are properly shot across the fluorescent surfaces of red, green and blue so that the fluorescent surfaces emit lights to display a predetermined image.
FIG. 2 is a front view of the conventional shadow mask. Referring to FIG. 2, conventional shadow mask 5 is a thin metal plate that has a lot of holes 51. More specifically, the holes 51 are aligned vertically on the thin metal plate and rows of the holes 51 aligned vertically are aligned horizontally. The electron beams pass through the holes 51. Invar mask or AK (Aluminum Killed) is used as the material of the shadow mask 5. The invar mask is trice as expensive as the AK. Both of them are critically different from each other in physical characteristics and are shown in Table 1.
TABLE 1MaterialINVARAKPriceHighLowDomingGoodBadEtchingBadGoodFormabilityBadGoodMain component (%)Fe: 64-60Fe: 99.7-99.0Ni: 35-36Thermal expansiveEqual to or less8 − 20 × 10−6/° C.coefficientthan 1.5 × 10−6/° C.
Referring to Table 1, AK is a pure iron that contains iron of 99.0%-99.7% and is inexpensive. However, its thermal expansive coefficient is 8-20×10−6/° C. and it is easier to be deformed than Invar. AK is as 5.3-13.3 times as Invar in their thermal expansive coefficients.
On the other hand, the doming means that the shadow mask 5 bulges due to heat. The heat is almost generated by the electron beams 11 striking the shadow mask 5 while the electron beams pass though the shadow mask 5. The degree of the doming determines the transmittance and the transmittance determines display quality.
The structure of the conventional shadow mask 5 will be described with reference to FIG. 2. The size and the shape of the conventional holes through which the electron beams pass are described. The structure of the conventional shadow mask satisfies the following relation:       B3    P3    ≤      B2    P2                  where B1 is a length of a bridge as a distance between holes 51 in a vertical direction at the center portion of the shadow mask 5,        B2 is a length of a bridge as a distance between holes 51 in a vertical direction at four corner portions of the shadow mask 5,        B3 is a length of a bridge as a distance between holes 51 at edge portions of the shadow mask in a direction of a long axis of the shadow mask 5,        P1 is a vertical pitch of the holes at the center portion of the shadow mask 5,        P2 is a vertical pitch of the holes at the four corner portions of the shadow mask 5, and        P3 is a vertical pitch of the holes at the edge portions of the shadow mask 5 in a direction of a long axis of the shadow mask 5.        
Considering that a pitch Ph is the same as a horizontal width S, the mask transmittance of the four corner portions is reduced. This lowers brightness and uniformity (the ratio of the peripheral portions to the brightness of the center portion). In such a structure, the doming is more serious at the area around the holes of the edge portions in the direction of the long axis than at the corner portions. The reason is as follows. It is known that doming is less as the thermal capacity per unit area is larger. However, the conventional shadow mask that is designed to satisfy the condition       B3    P3    ≤      B2    P2  has the bridge B2 at the diagonal corner portions to be greater than the bridge at the edge portion in the direction of the long axis. In this structure, since the thermal capacity at the corner portions in the direction of the long axis is forced to be comparatively less, the doming due to the electron beams is greater at diagonal corner portions. Because of the problems described above, in order to reduce doming, the Invar mask has been used in the conventional shadow mask 5 even though the Invar mask is expensive but the defect in the structure is still raised as a problem. Accordingly, it is required to suggest the shadow mask that overcomes the structural defect of the conventional shadow mask, is less expensive and has improved doming characteristics.
On the other hand, the outer surface of the effective portion is tended to be flat in order to improve affirmation. This requires for making the effective surface of the shadow mask 5 corresponding to the fluorescent screen installed on the inner surface of the effective portion flat. However, if the curvature radius of the shadow mask 5 is simply increased to make it flat, doming is caused since the shadow mask 5 is locally expanded by heat very greatly due to the collision of electron beams 11 of the high density. The doming makes the color purity degenerated.
To overcome this problem, the curvature radius in a direction of a long axis of the shadow mask is reduced to increase the curved sustain strength and suppress the doming. However, the curvature radius of the shadow mask cannot be small limitlessly since the curvature of the shadow mask relates to the curvature of the inner surface of the panel. If the curvature radius of the shadow mask is reduced, the curvature radius of the panel is also reduced. If the peripheral portions are thicker than the center portion by the amount more than some threshold, transmittance of the peripheral portions is reduced to lower the brightness of the peripheral portions and affirmation.
In this context, the proper curvature radius of the shadow mask should be suggested.
On the other hand, the curved surface strength of the shadow mask 5 is weakened due to the flatness of the shadow mask even though the shadow mask made of INVAR material is used in order to suppress the doming. Color cathode ray tube is easily damaged by an external impact. It is not wondering that the cathode ray tube including the shadow mask made of expensive INVAR material is very expensive.
Accordingly, the present invention is directed to a shadow mask for a color cathode tube that substantially obviate one or more problems due to limitations and disadvantages of the related art.
The present invention is suggested to overcome the above-mentioned problems. An object of the present invention is to provide a shadow mask for a color cathode ray tube to make the brightness of the corner portions proper, improve the bright uniformity of the entire screen and reduce doming at the edge portions in the direction of a long axis by improving the structure of shadow mask.
Another object of the present invention is to provide a shadow mask for a color cathode ray tube capable of reducing superior doming characteristic even though AK material is used for the shadow mask.
A further object of the present invention is to provide a shadow mask for a cathode ray tube to reduce doming effect, enhance the maintenance strength of the curved surface and provide the curvature radius of the shadow mask so that the ratio of the thickness of the peripheral portion to the thickness of the center portion is proper.